MFTV0005: Fix or remove the invalid value tag annotation

Cause

A [ValueTag] attribute or a ValueTag comment cannot be applied, so it tags nothing:

public sealed class Shipment
{
    [ValueTag("")] public Guid Id { get; set; }                         // MFTV0005, the tag is empty
    [ValueTag(Key = "OrderId")] public Guid OrderId { get; set; }       // MFTV0005, Key only applies to dictionaries
    [field: ValueTag("CarrierId")] public Guid CarrierId { get; set; }  // MFTV0005, the backing field is not analyzed
    /* ValueTag=ProjectId */ public Guid ProjectId { get; set; }        // MFTV0005, comments only tag local variables

    public void M()
    {
        Guid /* ValueTag= */ id = Guid.NewGuid();                       // MFTV0005, the comment cannot be parsed
    }
}

The message explains what is wrong. The rule reports:

• an empty tag, or an attribute that declares no tag,
• Key and Value on something other than a dictionary, [ValueTag("Tag")] on a dictionary, and an attribute that combines tags with Key and Value,
• [field: ValueTag] on a property, whose backing field is not analyzed,
• an [assembly: ValueTag] attribute that does not name a member, that names a missing member, or that uses Key and Value, and the ValueTag(typeof(...), "Member", ...) form on something other than the assembly,
• a comment that cannot be parsed, or that does not tag a local variable: a comment on a field, inside an initializer, at the end of a line, or before a statement that declares no variable,
• a comment in the type arguments of a field, a parameter, a generic method, or a type that is neither a collection, a dictionary, nor a wrapper, and /* ValueTag Key=... */ in a type argument,
• a comment that another comment overrides for every variable it could tag, with other tags.

How to fix

Fix the annotation as the message explains:

public sealed class Shipment
{
    [ValueTag("ShipmentId")] public Guid Id { get; set; }
    [ValueTag("OrderId")] public Guid OrderId { get; set; }
    [ValueTag("CarrierId")] public Guid CarrierId { get; set; }
    [ValueTag("ProjectId")] public Guid ProjectId { get; set; }

    public void M()
    {
        Guid /* ValueTag=ShipmentId */ id = Guid.NewGuid();
    }
}

The code fix removes the annotation when removing it is safe. See Tagging values and Tagging local variables for the valid forms.